1. Field of the Invention
The present invention relates generally to die casting. More specifically, the invention relates to monitoring and controlling a die casting operation.
2. Description of the Related Art
Die casting is the injection of molten metal under high pressure into a steel mold, interchangeably referred to as a die, for the purposes of rapid manufacturing at rapid production rates. The molten metal is most often a non-ferrous alloy, which are used because the best performance for die-cast products is gained through a blend of materials. Some typical alloys that are used for die casting are aluminum alloys, magnesium alloys and zinc alloys, which contain other elements such as silicone.
Two methods can be used to inject molten metal into a die; cold chamber and hot chamber. A schematic illustration of a typical cold chamber die casting machine 500 is shown in FIG. 1A. The die casting machine 500 comprises a mold 502 made of tool steel in at least two die halves 504, 506 that together define a part cavity 508. The cover half 504 is held by a fixed machine platen 505, and the ejector half 506 is held by a moving machine platen 507 so that the ejector half 506 can move back and forth to open and close the mold 502. Molds 502 often also have moveable slides, cores, or other sections to produce holes, threads, and other desired shapes in the casting. Molds 502 are alternately referred to as dies or tools.
The die casting machine 500 further includes a pressure chamber 510 through which molten metal from a supply 512 is delivered or injected into the mold 502 using a plunger 514. One or more shot sleeves 516 in the cover half 504 allow molten metal to enter the die and fill the part cavity 508. When the pressure chamber 510 is filled with molten metal, the plunger 514 starts traveling forward and builds up pressure, thereby forcing the metal to flow though the shot sleeve 516 to the part cavity 508. After the metal has solidified, the plunger 514 returns to its initial position, and the ejector half 506 of the die opens for the part or casting to be removed from the mold 502. Ejector pins 517 are used to push the casting out of the ejector half 506 of the mold 502. This process is referred to as a single casting cycle. Multiple casting cycles can be completed during a die casting operation.
A schematic illustration of a typical cold chamber die casting mold 502 is shown in FIG. 1B. The die casting mold 502 comprises a biscuit 518, which is the remaining material in the shot sleeve 516 after the shot is complete. One or more runners 520 connect the shot sleeve 516 to corresponding gates 522 through which molten metal enters the part cavity 508. One or more overflows 524 are connected to the part cavity 508 to receive the first molten metal that enters the part cavity 508 because it is usually contaminated with petroliates from the die spray applied to the mold 502 in previous casting operations.
Cooling lines 526 run throughout the mold 502, through which coolant, such as water or oil flows to aid in the removal of heat from the mold 502. There are a number of individual cooling lines 526 that are responsible for cooling different parts of the casting or shot. The number of cooling lines 526 in a mold varies according to the size of the mold. For example, a small mold may have fifteen cooling lines, while a large mold may have over a hundred cooling lines. The cooling lines 526 are all in communication with a coolant flow system (not shown), from which coolant is delivered to the cooling lines, and to which coolant returns after it flow through the cooling lines. Many coolant flow systems for dies are part of a plant-wide water system. Other coolant flow systems are “closed-loop” systems, in which coolant is only cycled through the coolant flow system.
The casting can be divided into multiple heat flow zones that are cooled by one or more cooling lines 526. The heat flow zones are generally indicated by the dotted boxes on FIG. 1B. The heat flow zones of the casting comprise the biscuit (Zone 0), the main runner (Zone I), the gate runner (Zone II), the gate side of the casting (Zone III), the overflow side of the casting (Zone IV), and the overflow (Zone V). The biscuit (Zone 0) generally corresponds to the biscuit 518. The main runner (Zone I) corresponds to the portion of the runners 620 that are closest to the biscuit 518. The gate runner (Zone II) corresponds to the portion of the runners 520 that is closest to the part cavity 508. The gate side of the casting (Zone III) is the casting half nearest to the gates 522. The overflow side of the casting (Zone IV) is the casting half furthest away from the gates 522. The overflow (Zone V) generally corresponds to the overflows 524.
There are primarily three critical die-casting process control requirements. The first requirement relates to the timing and function of the die casting machine. The timing of the opening and closing of the mold must be closely managed during the process to sequence operations such as injecting metal into the part, dealing with moving slides, making any intricate details in the casting, and extracting the part. The timing of these and other operations can be controlled to optimize the production rate and quality of the castings.
The second requirement relates to the injection processes at the shot end of the die casting machine. The injection processes, both from the standpoint of hardware and software, have been developed over time to optimize the control of injecting the liquid metal into the mold. Injection speed, injection pressure, and flow rate are all involved in the control of the injection process and can be taken into account during the design of the die casting process. Technologies have been developed to address the first two requirements in terms of machine design and shot end design to manage the first two problems that die casters have dealt with.
The third requirement relates to the thermal design, monitoring and control of the die casting process, including temperature detection and the removal of heat from the mold. Thermal design encompasses designing the cooling system of a die casting machine, which includes determining the number of cooling lines, the placement of each cooling line relative to the part cavity, the depth of each cooling line relative to the die surface, using the appropriate size, i.e. diameter, of cooling line, and determining the appropriate flow rate of each cooling line. Thermal monitoring refers to monitoring temperature and heat during the actual use of the die. Thermal control encompasses taking the information gathered from thermal monitoring and responding to that information, with respect to the intended thermal design.
Thermal design has historically been haphazard in the engineering of die casting processes. This is partly because the mathematics involved in thermally designing a die can be complex.
Thermal monitoring and control has to be almost non-existent in the die casting industry, although a few attempts have been made in the field to monitor temperatures and flow rates. Some dies employ simple flow monitoring devices that are essentially mechanical flow meters to monitor the flow rate of coolant through cooling lines.
From a theoretical standpoint, the thermocouples can be used to determine the die surface temperature. Typically, a thermocouple is placed by drilling a hole to a location between the die surface and the cooling line surface, usually approximately halfway between the die surface and the cooling line surface. In use, the die surface temperature may be as high as 700 to 800 degrees Fahrenheit, while the cooling line surface temperature may be 100 degrees, and there may be less than one inch between the die surface and the water line surface. Therefore, a steep thermal gradient exists between the die surface and the water line surface, and the thermocouple is located within this steep thermal gradient. The location of the thermocouple within the temperature gradient, i.e. the distance of the thermocouple from the die surface, is used to determine the temperature at the die surface.
One problem with using thermocouples to monitor temperature lies in accurately placing the thermocouple at a desired location. Thus far, thermocouples have proved unreliable in determining the die surface temperature. Because it is difficult to drill in a straight line though the mold, it is almost impossible to know the exact location of the thermocouple within the temperature gradient. This is highly undesired, since even small deviations from the planned location of the thermocouple can result in large inaccuracies in temperature. For example, if the end of the drilled hole is off by 1/10 inch in either direction, the location of the thermocouple within the temperature gradient may cause a ±25 to 50 degree Fahrenheit variation in the temperature measured.
Another problem associated with using thermocouples to monitor temperature are in their physical functionality. Thermocouples require adequate contact with the mold for accurate thermal measurement, but thermocouples are often difficult to seat properly within the drilled hole. J- and K-type thermocouples, the type of thermocouples used in die casting processes, do not have a high level of accuracy when it comes to die casting process, because they have a read error from 1 to 2.5%. Thermocouples often can break and must be replaced. Thermocouples have wires that come out of the die that must be plugged into a box to measure the temperature from the thermocouple, and these wires can be easily cut or otherwise damaged. Die setup can vary from 30 minutes to eight hours, and the list of items that must be completed in the setup is on the order of 30 to 100 different specific things that must be done to remove a mold and put a new mold into the die. Adding to that process by having to connect and verify the function of thermocouples is not very desirable.
Yet another problem associated with using thermocouples to monitor temperature is that thermocouples can only be used in select areas within the mold. Areas such as the biscuit, the runner system, the overflows, and slides cannot be fitted with thermocouples, and so the temperature of these areas of the mold are not monitored.
Thermal control using information supplied by thermocouples in past die casting systems has been rudimentary at best. The data supplied by thermocouples can be tracked and used for correlation with product quality. Some die casting systems are configured to turn coolant flow on or off based upon thermocouple readings, in which case there is no respect for the heat removed from the mold. One issue with this practice is that it can induce some thermal variation into the die casting process because there is a lag between the temperature the thermocouples are detecting and the temperature at the surface of the mold. Turning coolant flow on and off creates a sinusoidal temperature variation at the mold surface.
Another problem with current die casting thermal monitoring, and control is that little emphasis has been given to dimensional accuracy and precision in relation to gas porosity defects. The die casting process has long been considered a net shape process, but not an accurate one. The reason behind the poor dimensional accuracy and precision is that the injection temperature of the liquid metal varies in different sections of the casting, and the casting is ejected at an inconsistent temperature, the shrinkage that the casting undergoes will be inconsistent as well since the entire casting has to cool down to ambient temperature. For example, if one section of the casting is at a temperature of 800 degrees Fahrenheit at ejection, and another section of the casting is at 300 degrees Fahrenheit at ejection, the section at 800 degrees Fahrenheit will undergo more shrinkage than the section at 300 degrees Fahrenheit. This inconsistent shrinkage will create distortion and dimensional inaccuracy in the casting, which will force the utilization of machining operations to achieve reasonable dimensional control.
Another problem associated with poor thermal monitoring and control occurs during the process of ejecting the casting from the mold. If there is a “hot spot” in the die, i.e. a portion of the die that retains more heat than the rest of the die, ejection is delayed because that areas of the casting must cool longer than the rest of the casting, which means that the remainder of the casting will be cooler than it needs to be for ejection. When the casting cools too long within the mold, it can contract around details in the die, and may then require significant force to eject the casting, which can cause distortion or cracking of the casting. Waiting for the portions of the casting near the “hot spots” to cool also results in longer cycle times.
“Hot spots” in the die may also cause soldering to occur, which is when the temperature of a portion of the die is so high that the die spray burns off and the casting sticks inside the part cavity. The casting may still be ejected, but some of the casting material may stick to the die and oxidize.